From 2d4151ddfea8024b3cc7e58ff9172d1f7b5cadab Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 24 May 2016 23:54:16 +0200
Subject: [PATCH] Update containers when Items changes.

---
 .../Presenters/ItemVirtualizerSimple.cs            | 12 ++++++++++++
 .../ItemsPresenterTests_Virtualization.cs          | 14 ++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs b/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
index afbd501fc2..32fa967793 100644
--- a/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
+++ b/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
@@ -89,6 +89,18 @@ namespace Avalonia.Controls.Presenters
         public override void ItemsChanged(IEnumerable items, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsChanged(items, e);
+
+            if (e.Action == NotifyCollectionChangedAction.Reset)
+            {
+                // We could recycle items here if this proves to be inefficient, but
+                // Reset indicates a large change and should (?) be quite rare.
+                VirtualizingPanel.Children.Clear();
+                Owner.ItemContainerGenerator.Clear();
+                FirstIndex = 0;
+                LastIndex = -1;
+                CreateRemoveContainers();
+            }
+
             ((ILogicalScrollable)Owner).InvalidateScroll();
         }
 
diff --git a/tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization.cs b/tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization.cs
index a3fc24583c..3275c1bfba 100644
--- a/tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization.cs
+++ b/tests/Avalonia.Controls.UnitTests/Presenters/ItemsPresenterTests_Virtualization.cs
@@ -184,6 +184,20 @@ namespace Avalonia.Controls.UnitTests.Presenters
                 Assert.Equal(8, target.Panel.Children.Count);
             }
 
+            [Fact]
+            public void Should_Update_Containers_When_Items_Changes()
+            {
+                var target = CreateTarget();
+
+                target.ApplyTemplate();
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(0, 0, 100, 100));
+
+                target.Items = new[] { "foo", "bar", "baz" };
+
+                Assert.Equal(3, target.Panel.Children.Count);
+            }
+
             public class WithContainers
             {
                 [Fact]